A conventional multiplexer receives data from a plurality of data sources or data terminal equipment (DTE) and combines the data for transmission over a single high speed communication channel. In order to accommodate the data throughput demand, the communication channel capacity must at least equal the aggregate data rate from the data sources.
Bit multiplexers operate on received data on a bit by bit basis. These multiplexers require that the ratio of the aggregate data rate to the signalling (baud) rate be an integer. Also, the ratio of the bit rate of each DTE to the baud rate must be an integer. It will be apparent that these conditions place restrictions on the bit rates and baud rates which can be used.
Statistical multiplexers require that the aggregate transmission rate be equal to the sum of a subset of DTE port rates. For example, if DTE rates were 14.4, 2.4 and 2.4 Kbps, the statistical multiplexer would require the communication channel to be able to support at least 14.4 Kbps. Thus, the highest DTE rate could not be accommodated by splitting the throughput demand over two or more communication channels.
Multiplexing systems have used a feature known as restoral. Systems with this feature provide a means for switching to an alternate transmission channel upon the failure of the main communication channel or degradation below the maximum throughput rate required. If conditions dictate switching to an alternate channel, the restoral feature switches the entire data stream to an alternate channel thereby abandoning all communications over the main channel.
Biplexers generate transmission data streams with equal data rates. Thus, they cannot take full advantage of transmission channels with different throughput capabilities. Normally biplexers split a user data stream into multiple data streams which are transmitted over separate transmission devices. Any change in the data rate or failure of a channel will result in a drop in the available user data rate.